Just below the given answer there is a box, stating "Type your comment to this answer here".You can use that box to to collaborate on tasks - ask for more details, ask additional questions before giving an answer, etc. They can also be used to show your appreciation to the person who helped you.Just click on the "send" button or press "enter" to post your comment.
To answer questions it's best to use the answering box than post it in comments as those are length-limited and don't grant you points.
Hope you've understood ! :)